1. Field of the Invention
The present invention relates to a method for assembling a nuclear fuel assembly, and an apparatus for performing such a task.
2. Background Art
Fuel assemblies such as the one disclosed in U.S. Pat. NO. 5,068,081 shown in FIG. 7 have been known.
In this figure, the numerals 1 and 2 refer to a top and a bottom nozzles, respectively, which are disposed vertically and oppositely spaced apart, and between the top nozzle 1 and the bottom nozzle 2 are a plurality of rigidly fixed control-rod guide pipes 3 (hereinbelow referred to as guide pipes 3), and in the midway section of the control-rod guide pipes 3 are a plurality of grids 4 disposed vertically and spaced apart from each other.
The grids 4 are, as shown in FIGS. 8, 9 and 10, constructed of a plurality of straps 7 made of thin metal strips which have slits 8 provided at regular intervals in the longitudinal direction. By inserting the straps 7 into the slits 8 in each other's slits, a plurality of grid cell spaces 5 (hereinbelow referred to as grid cells 5) are formed. The fuel rod 6 is held firmly in the grid 5, by means of a pair of holding means consisting of a dimple 9 and a spring 10 disposed on opposing walls. The fuel rod 6 inserted into a grid cell 5 is pressed against the dimples 9 by the springs 10 as illustrated in FIG. 10, thereby supporting the fuel rod 6 firmly by the springs 10.
The conventional method of assembling such a fuel assembly will be described with reference to FIG. 11.
First, the grids 4 are placed apart with a certain distance (positioning of grids). The guide pipes 3 are inserted into the predetermined and corresponding grid cells 5 of each of the grids 4, and are then rigidly attached to the grids 4 by bulge forming. Subsequently, the fuel rods 6 are inserted into the predetermined and corresponding grid cells 5 of each of the grids 4, by sliding the rods 6 against the springs 10 and dimples 9. The rods 6 are thereby secured in the grid cells 5 via the pressing force exerted by the springs 10 and dimples 9. When all of the rods 6 are inserted into the grid cells 5, the top and bottom nozzles 1 and 2 are firmly mounted to the opposite ends of the respective guide pipes 3.
In the conventional assembling method described above, the rods 6 are inserted, by sliding over the holding device (composed of dimples 9 and springs 10 ), into the spaces of the grid cells 5 after the guide pipes 3 have been rigidly attached to the grids 4. Because of the procedure, the guide pipes 3 are placed under slight non-uniform compressive stresses transmitted from the fuel rods 6 through to the grids 4. Further detailed explanations are provided in the preferred embodiments of the invention, but it suffices to mention that one of the contributions of the present invention is that the completed assembly shows very little distortion, as a result of the invented process of assembling the fuel assembly.